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1==========================2Understanding fbdev's cmap3==========================4 5These notes explain how X's dix layer uses fbdev's cmap structures.6 7- example of relevant structures in fbdev as used for a 3-bit grayscale cmap::8 9 struct fb_var_screeninfo {10 .bits_per_pixel = 8,11 .grayscale = 1,12 .red = { 4, 3, 0 },13 .green = { 0, 0, 0 },14 .blue = { 0, 0, 0 },15 }16 struct fb_fix_screeninfo {17 .visual = FB_VISUAL_STATIC_PSEUDOCOLOR,18 }19 for (i = 0; i < 8; i++)20 info->cmap.red[i] = (((2*i)+1)*(0xFFFF))/16;21 memcpy(info->cmap.green, info->cmap.red, sizeof(u16)*8);22 memcpy(info->cmap.blue, info->cmap.red, sizeof(u16)*8);23 24- X11 apps do something like the following when trying to use grayscale::25 26 for (i=0; i < 8; i++) {27 char colorspec[64];28 memset(colorspec,0,64);29 sprintf(colorspec, "rgb:%x/%x/%x", i*36,i*36,i*36);30 if (!XParseColor(outputDisplay, testColormap, colorspec, &wantedColor))31 printf("Can't get color %s\n",colorspec);32 XAllocColor(outputDisplay, testColormap, &wantedColor);33 grays[i] = wantedColor;34 }35 36There's also named equivalents like gray1..x provided you have an rgb.txt.37 38Somewhere in X's callchain, this results in a call to X code that handles the39colormap. For example, Xfbdev hits the following:40 41xc-011010/programs/Xserver/dix/colormap.c::42 43 FindBestPixel(pentFirst, size, prgb, channel)44 45 dr = (long) pent->co.local.red - prgb->red;46 dg = (long) pent->co.local.green - prgb->green;47 db = (long) pent->co.local.blue - prgb->blue;48 sq = dr * dr;49 UnsignedToBigNum (sq, &sum);50 BigNumAdd (&sum, &temp, &sum);51 52co.local.red are entries that were brought in through FBIOGETCMAP which come53directly from the info->cmap.red that was listed above. The prgb is the rgb54that the app wants to match to. The above code is doing what looks like a least55squares matching function. That's why the cmap entries can't be set to the left56hand side boundaries of a color range.57